The United States' ultra-secret National Security Agency reluctantly finds itself in headlines amid a wave of disclosures from ex-intelligence contractor Edward Snowden, who has exposed the service's vast electronic spying operations.

France and Mexico both demanded explanations Monday after the latest revelations from Snowden alleged the NSA secretly monitored tens of millions of phone communications in France and hacked into former Mexican president Felipe Calderon's e-mail account.

Hollywood directors and novelists have made the US' Central Intelligence Agency famous for its undercover agents in the field, but in the digital era, the high-tech NSA may represent the most far-reaching arm of the country's 16 spy agencies, with its intelligence at the center of decision-making and military planning.

The agency uses super computers, linguists and code-breaking mathematicians to oversee what experts say is the world's most powerful digital espionage organization, scooping up phone conversations and e-mail traffic relevant to "foreign targets".

Created after World War II to avoid another Pearl Harbor-style surprise attack, the NSA "has transformed itself into the largest, most covert, and potentially most intrusive intelligence agency ever created," wrote author James Bamford, whose books helped lift the lid on the agency's work.

AFP